Sample: falloff for spotlights in simplePBR (3.7 KB)
assuming you’re already on simplePBR and have a spotlight… I’ve tweaked the shaders to do falloff as described at

you don’t even need a custom build of simpleBPR. you can grab the shaders from the zip file or the pull-request, and then drop them into <your_project_dir>/shaders

        spot = light.node()
        lens = spot.get_lens()
        lens.set_fov(self.state.angle * 2) 
        # dirty. use the previously unused-by-pbr 'exponent' field to pass the PBR falloff angle to the shader

works with or without shadows